11 March 2017

Coarse threads vs Fine threads

The threads are divided into two categories such as coarse and fine series. The thread profiles of both the thread are generally similar but there has some minor difference between them. Let us have a deep insight into the comparison between coarse and fine threads. 

Both the screw threads are designated by the letter M followed by the value of the nominal diameter in mm. Example: M 12 

Difference : 
  • Course thread has higher peaks and valleys than fine thread making them harder to strip.
  • Fine thread is harder to loosen by vibration to its tighter coiling and they also require less torque to achieve the same holding power as a coarse thread would.  
  • The height of coarse thread is greater than the corresponding fine thread so there is more material between each thread-making flank engagement greater.
  • Coarse threads are less susceptible to being nicked or damaged, so they do not have to be handled with care as much as fine threads. 
  • Coarse threaded fasteners are much faster to install than fine threaded fasteners.
  • Coarse threads are not affected as much as fine threads by plating buildup.
  • Fine threaded bolts of the same hardness are stronger than the corresponding coarse threaded bolts.
  • Due to their smaller helix angle than coarse threads, fine threads tend to loosen under vibration.
  • To develop equivalent preloads to the corresponding coarse thread bolt sizes, fine threads require less tightening torque.

Comparison according to advantages : 

Advantages of coarse threads :
  • Easier to cut as compared to fine threads.
  • It has more even stress distribution.
  • During tightening, coarse threads are less likely to seize.
  • The capacity of coarse threads to carry static load is higher.
  • The manufacturing error is lower in coarse threads.
  • Wear has less effect than fine threads on the strength of coarse threads.
Advantages of fine threads :
  • Greater strength when subjected to fluctuating loads.
  • Because of the lower helix angle, greater resistance to unscrewing.
  • More reliable in terms of self-unscrewing than coarse threads.

Summary : 

Coarse threads are used for general industrial applications that are vibrations free while fine threads are used in the parts that are subjected to dynamic loads and vibrations.